1. Field of the Invention
This invention relates generally to optoelectronic devices and, more particularly, to infrared photodiode photodetectors.
2. Description of the Related Art
Photodetectors are used in numerous applications to detect light and provide a corresponding electrical signal. Infrared (IR) photodetectors are one class of detectors which are included in a variety of applications, such as night vision, communications, and environmental monitoring. IR detectors can be based on several different material systems, including silicon (Si), gallium arsenide (GaAs), silicon germanium (SiGe), aluminum gallium arsenide (AlGaAs), lead salts (PbS, PbSeTe, PbSnTe), various Hg-bearing compounds, pseudobinary alloys of HgTe and HgSe with CdTe, CdSe, MnTe, MnSe, ZnTe, and ZnSe. Mercury cadmium telluride (HgCdTe or MCT) is attractive because it has a direct energy gap, can be grown as high-quality epitaxial thin films on transparent substrates, can be doped to obtain both high and low carrier concentrations, and spans the entire IR wavelength range from ˜0.8 μm to >20 μm. It can also be compositionally graded to vary the bandgap energy with position. More information regarding MCT photodetectors can be found, for example, in U.S. Pat. No. 6,034,407.
Photodiodes provide the highest performance of all photodetectors. A photodiode consists of a semiconductor pn junction. The semiconductor absorbs light having photon energy higher than the semiconductor's bandgap, creating electron-hole pairs. If the light is absorbed in the n(p)-region, the holes (electrons) are the minority carriers and diffuse to the depletion region, where they are swept by the depletion region electric field into the p(n)-region to create a photocurrent that becomes the detector signal. Absorption of light in the depletion region produces this sweeping without need for diffusion.
The fundamental quality of a detector is its signal-to-noise ratio (SNR); i.e., the ratio of the photocurrent to the noise current present in the detector under measurement conditions. The highest quality a detector can have in any operating condition is to have a SNR limited by the noise current that comes from the photon background itself. This condition is called “background-limited performance” or BLIP. The highest BLIP quality is attained when every absorbed photon generates a minority carrier that contributes to the photocurrent. The efficiency of minority carrier collection is called the quantum efficiency, and perfect detectors will have a quantum efficiency of 1. Note that if the light is absorbed in a material of relatively low bandgap, but the pn-junction is in a portion of the semiconductor structure with a wide bandgap (i.e., if the structure is a type of “heterostructure” having spatially varying energy gaps), under some conditions an energy barrier exists that frustrates diffusion of minority carriers to the depletion region and significantly reduces the photocurrent.
Current MCT detector technology does not operate near its full potential. One reason is because of point and extended defects in the material, which can be caused by impurities or vacancies, imperfect surfaces, interfaces, or damage introduced during fabrication. These defects can cause recombination of any minority carrier in their vicinity, in particular those photogenerated carriers that produce the detector signal. This recombination decreases the detector quantum efficiency.
The defects mentioned above may also cause the detectors to have too much noise. Dark current, which is a current that flows through the photodetector in the absence of incident light, adds noise to that inherent in the photocurrent, lowering the SNR below the BLIP level. The dark current is caused by the thermal generation or tunneling of charge carriers due to fundamental mechanisms, to point defects, or to extended defects. It is generally desired to have the photodetector provide its maximum (BLIP) sensitivity at as high at temperature as possible, up to room temperature to avoid the need for elaborate cooling schemes. However, the thermally generated dark current-induced noise typically increases exponentially with increasing operating temperature. Consequently, there is a need for a photodetector with fewer defects near the active region so that, at a given temperature, the quantum efficiency is improved and the defect-generated noise is reduced.
Dark currents are lower in materials with higher (“wider”) energy gaps than those in materials with lower (“narrower”) gaps. This is because the defects can only generate dark currents in proportion to the equilibrium concentration of minority carriers in the semiconductor, and because this concentration for fundamental physical reasons is exponentially lower in higher energy gap materials. Therefore, one expects the dark currents from defects to be reduced greatly in wide bandgap materials compared to their currents in narrow gap materials. Roughly, for a given level of defect-moderated and fundamental mechanisms, the same dark current density will be realized for a given ratio of energy gap to temperature.